The present invention relates to fasteners in general, and, more in particular, to female fasteners or nuts of the torque-limiting type.
In a standard threaded fastener system of a male threaded fastener and a female threaded fastener, the female fastener has internal threads that thread onto external threads of the male fastener. Wrenching surfaces of both fasteners accept tools that tighten them and clamp one or more workpieces together between them oftentimes with washers interposed in between. The combination of the fasteners and the workpieces are known as a "joint." Male threaded fasteners are variously known as "studs," "screws," "bolts," or "pins;" female threaded fasteners are variously known as "nuts" or "collars;" workpieces are sometimes called "sheets" or "structural elements."
Fasteners bear loads along their axes, tensile loads, and radially of the axes, shear loads. Tensile loading always exists because of the clamping force applied by the pin and the collar to the sheets; this load is known as "clamp-up" or "preload." When fasteners join two or more sheets and the sheets are loaded in their planes, one sheet may tend to slide over the other; when this loading of the sheets occurs, it is resisted by the fasteners, and the sheets load the fasteners in shear. Shear loads are transverse to the axes of the fasteners and transverse to the tension load. Cyclic loading of a fastener can produce fatigue failure. In aerospace applications fatigue failure is usually most critical in shear.
Adequate clamp-up or preload is absolutely necessary for a satisfactory joint A fastener adequately loaded by the reaction to the clamp-up load resists fatigue failure. Preload also helps the structural elements to resist fatigue failure. Accordingly, it is desirable to know the clamp-up load the fastener applies to a structure to be sure that a joint has adequate fatigue strength. Adequate clamp-up also avoids sheet slippage and fretting and insures against load shifting and joint failure.
Clamp-up load correlates to the resistance of a collar to further threading onto a pin and against a workpiece by the application of torque to the collar. As clamp-up force increases, the resistance to further threading increases, and the torque required to turn the collar increases.
This fact has been used in fasteners to develop a predetermined clamp-up load by termination of tightening through failure of a wrenching section on the collar. U.S. Pat. No. 2,940,495 to G. S. Wing and U.S. Pat. No. 4,260,005 to Edgar Stencel describe two types of such fasteners.
The Wing patent describes a collar extensively used in the aerospace industry. It has a wrenching section connected to an internally threaded section by a frangible breakneck collar. The collar breaks upon the application of a predetermined torque that corresponds to a desired clamp-up load. An acircular portion of the threaded section provides a thread lock by pressing tightly against the threads of the cooperating pin. A problem with this type of fastener is that it generates a waste piece: the wrenching section. The waste piece must be removed from the environment where the fastener is set. This type of fastener is also comparatively expensive because it requires a considerable amount of machining to make it, and the frangible breakneck must be held to very close tolerances to provide close tolerances in breakoff torques.
The Stencel patent describes a collar that has a plurality of circumferentially spaced lobes on its axial outside that serve as wrenching surfaces and in torque limitation. A wrenching tool, say a triangular shaped socket, has flats that engage flanks of the lobes and turn the collar with respect to the pin. Upon reaching a predetermined clamp-up load, the lobes fail in radial compression and merge into the body of the collar, and wrenching and tightening stops because the lobes no longer provide purchase for the setting tool. The Stencel collar produces a thread lock by a deformation of collar material radially inward of the lobes against the threads of a cooperating pin when the lobes fail.
Impact wrenches used in setting fasteners do so rapidly. The failures of the breakneck of the Wing fastener and of the lobes of the Stencel fastener occur over very few degrees of rotation, and, when an impact wrench is used, occur very rapidly. The rapid application of setting torques to a collar can result in loss of some desired preload through relaxation of the sheets; relaxation results from the continued deformation of the sheets after the initial loading. Sheet relaxation usually happens as a result of more than one fastener being necessary to pull all the parts together. Such deformation reduces the load per unit area and absolute loading because material moves away from the clamped zone. When the breakneck or the lobes fail, they fail at a torque corresponding to a desired preload. But the loaded sheets can relax and some of the preload lost. This relaxation is a time-dependent phenomenon, and with slower development of preload, relaxation and loss of preload will be less.
It may also be desirable to be able to change the preload even with the same collar. For example, when the sheets are not as strong in compression as some other sheets, it may be necessary to lower the compressive load on them.
In some applications secondary wrenching is desired in order to increase preload above design preload or to compensate for relaxation. Secondary wrenching is impossible in the standard configurations of the Wing and Stencel collars. These collars are also difficult to remove after they have been set because of the absence of wrenching sections.
In some applications because of space limitations straight-on wrenching is impossible and wrenching must be done from the side of the collar, say, with an open end wrench.